This invention relates to ramjet engines and to a nozzle structure for use therewith. More particularly, this invention concerns itself with a swing disc, variable nozzle structure for use in a ramjet engine.
One of the simplest air-breathing, aircraft, power plants is the ramjet engine. Because of its simplicity, it is also sometimes referred to as the "flying stovepipe". Ramjets are somewhat similiar to a turbojet engine without a compressor or a turbine. During operation of the ramjet, compression is obtained through the forward operation of the aircraft during which air is forced into the front of the engine through a diffuser at high velocity. This creates a dynamic pressure or "ram" in the diverging inlet section of the engine. The diffuser is shaped to reduce airspeed and its kinetic energy. Reducing its kinetic energy results in an increase in potential energy in the form of increased air pressure. The higher air pressure enters the combustion chamber of the engine and reacts with an injected fuel to create hot gases which are ejected rearwardly through the engine to provide propulsion.
The ramjet has no thrust at takeoff because at zero flight mach number there is no increase in air pressure through the diffuser. Since the engine has no thrust at takeoff and only low or minimal thrust at low speeds, other means or engines are relied upon to provide a takeoff boost. For example, rockets are used as a booster for missiles while aircraft often use a tubojet as a means for assisting in takeoff. One of the distinct advantages of a ramjet lies in the fact that as the flight speed of the ramjet increases, the pressure ratio and engine efficiency likewise increase with an increase in thrust and a very desirable resulting decrease in fuel consumption.
Another advantage of a ramjet engine is the simplicity provided by its fixed geometry. This simplicity, however, is very deceiving since the pressure, temperature, velocity and volumn of the ejected hot gases all vary as they progress through the ramjet. Also, the airflow passage areas of the engine have precisely calculated values depending on flight conditions, operational altitudes and thrust requirements. If any of these flight values change, the calculated values change also. It becomes apparent, therefore, that if a ramjet engine is to operate efficiently under variable flight conditions and differing altitudes, then a compromise must be made in design characteristics.
The usual compromise is to designate one flight condition as the design point and accept degraded or non-optimum performance at all other flight conditions. For modern ramjets that are expected to operate over a large range of Mach numbers and altitudes this can be a very significant compromise. An alternative approach is to vary the physical geometry of the engine to provide on-design performance at all flight conditions. This, of course, also has severe limitations; generally it is practical only to vary the flow areas of the ramjet inlet and nozzle. Even this limited variation can become quite complicated, however, and it is necessary to examine the design and the application in detail to determine the type and amount of variable geometry that will provide the desired result.
For example, the Advanced Strategic Air Launched Missile (ASALM) engine is a multi-purpose integral rocket ramjet designed for operation from about Mach 2.0 at low altitudes to Mach 4.5 at high altitudes. The missile is boosted from aircraft launch velocity by an integral solid rocket to about Mach 2.0 for ramjet takeover. The ramjet engine continues the missile acceleration to the cruise condition, and then continues operation for the cruise mission. However, the missile, which is designed to fit in the restricted boundaries of the B-1 rotary launcher, is severely volume limited. This limitation creates the need to obtain maximum performance from the ramjet engine to meet mission requirements. However, if the engine geometry is designed to provide maximum performance at the ramjet takeover Mach number, then the performance at the cruise Mach number will be far from optimum. Typically, the inlet may operate 30% supercritical at the cruise condition.
With awareness of this problem, it became obvious that a variable geometry engine would overcome, or at least significantly minimize, the effects of the problem. A considerable research effect was undertaken, therefore, with the resultant testing of numerous inlet and nozzle designs. Early in the research program it became apparent that because of the severe volume limitations and the relatively short engine operating times (typically 10 minutes), the geometry variation mechanism would need to be quite simple. It was also determined that discrete positioning components with two or three positions would provide nearly as much benefit as continuously varying components. Another significant fact discerned was that a variable nozzle alone can provide significant benefit, while a variable inlet alone provides only minimal benefit. As a result, a two-position swing disc nozzle structure was designed and subjected to further study and test evaluation. The study indicated that incorporation of this component in a 15-inch diameter multi-purpose missile potentially doubled its range. Wind tunnel tests of the inlet and cold flow performance tests of the nozzle were performed. Variable nozzle performance was most promising. Since the variable nozzle, by itself, provided a significant advantage for the ASALM engine.
The swing disc nozzle structure comprised a disc-shaped body positioned within the nozzle of a ramjet engine to form oppositely disposed throat sections for air passageways. Unfortunately, the severe operational conditions which exist within the ramjet engine, cause a considerable amount of oxidation to take place. This results in corrosion and erosion of the leading edge surfaces of the disc structure.
With the present invention, however, it was found that the degradative effects of the oxidizing conditions could be overcome by providing the disc assembly with coated graphite inserts positioned in recessed areas to form the leading edge of the swing disc. Coatings for the graphite insert can be selected from corrosion resistant hafnium, zirconium and silicon carbide coating materials which are applied to the graphite insert in accordance with conventional coating methods. The resulting disc nozzle structure was found to resist the effects of the corrosive and erosion condition, thus maintaining the design contour of the nozzle's throat areas during flight.